Hydroxyapatite (HAp) for Biomedical Applications

Michael Mucalo

• Publisher: Woodhead Publishing
• Year: 2015
• Print ISBN: 9781782420330
• eBook ISBN: 9781782420415
• Topic: Medicine
• Subtopic: Medical Technology & Supplies

Hydroxyapatite in the form of hydroxycarbonate apatite is the principal mineral component of bone tissue in mammals. In Bioceramics, it is classed as a bioactive material, which means bone tissue grows directly on it when placed in apposition without intervening fibrous tissue. Hydroxyapatite is hence commonly used as bone grafts, fillers and as coatings for metal implants. This important book provides an overview of the most recent research and developments involving hydroxyapatite as a key material in medicine and its application. - Reviews the important properties of hydroxyapatite as a biomaterial - Considers a range of specific forms of the material and their advantages - Reviews a range of specific medical applications for this important material

Part One

Properties and biological response to hydroxyapatite for medical applications

1

Structure and properties of hydroxyapatite for biomedical applications

K. Lin; J. Chang [email protected] Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, China

Abstract

Hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂, (HAp)] materials have attracted great interest from researchers because they are widely applied as biomedical materials due to their excellent biocompatibility, osteoconductive properties, and similarity to the inorganic component of human beings. In this chapter, the key properties of HAp will be briefly discussed, including the chemical composition, crystal structure and ion substitution, crystal growth kinetics and preferred orientation, charge distribution of the crystal faces, general aggregates of the crystals in the synthesis process, and relationship between the structures and properties. Then, the weaknesses and applications of HAp in biomedical applications were further reviewed. Moreover, the strategies to overcome the weaknesses were summarized and proposed. More to the point, the possible directions of future research and development in this field are proposed.

Keywords

Hydroxyapatite

Structure

Properties

Biomedical applications

1.1 Introduction: key properties

Hydroxyapatite (HAp) materials have drawn great interest from researchers because they are widely applied as biomedical materials, including such uses as bone fillers (Valletregi, 2004; Dorozhkin, 2009a; Xia et al., 2013), bone tissue engineering scaffolds (Dorozhkin, 2009b), bioactive coatings (Valletregi, 2004), soft tissue repairs (Okabayashi et al., 2009; Shin et al., 1992; Ji et al., 2012; Liu et al., 2012a), drug/protein/gene loading and delivery systems (Uskokovic and Uskokovic, 2011; Rodriguez-Ruiz et al., 2013; Lin et al., 2013a,b, 2011a; Zhu et al., 2004; Li et al., 2010), and column chromatography for rapid fractionation of biomolecules (Hilbrig and Freitag, 2012; Morrison et al., 2011) because of their excellent biocompatibility, osteoconductive properties, and similarity to the inorganic component of human bones (Dorozhkin, 2009b). HAp materials are also potential candidates for use in cell targeting, fluorescence labeling, imaging and diagnosis materials (Kozlova et al., 2012; Chen et al., 2012), etc. In addition, HAp is also exploited as a model compound to mimic the biomineralization process (Sadat-Shojai et al., 2013; Cai and Tang, 2008; Li et al., 2008).

Pure HAp is a stoichiometric apatite phase with a Ca/P molar ratio of 1.67 the most stable calcium phosphate salt at normal temperatures and pH between 4 and 12 (Koutsopoulos, 2002). The crystal structure of HAp most frequently encountered is hexagonal, having the P6₃/m space group symmetry with lattice parameters of a = b = 9.432, c = 6.881 Å, and γ = 120°. The structure consists of an array of PO₄ tetrahedra held together by Ca ions interspersed among them. The Ca ions occur in two markedly different sites, in accurately aligned columns (Ca(I)) and in axes, and the adjacent OHs point in opposite directions (Ma and Liu, 2009). In the P6₃/m form, the unit cells of HAp are arranged along the c-axis. This would justify a preferred orientation that gives rise to an oriented growth along the c-axis and a needle-like morphology (Valletregi, 2004). HAp can also exist in another form, i.e., the monoclinic form with space group P2₁/b and lattice parameters a = 9.4214(8), b = 2a, c = 6.8814(7) Å, γ = 120°. The major difference between the monoclinic and hexagonal HAp is the orientations of the hydroxyl groups (OHs). In monoclinic HAp, all of OHs in a given column are pointed in the same direction, and the direction reverses in the next column, while in hexagonal HAp the adjacent OHs point in opposite directions as mentioned above (Ma and Liu, 2009). The hexagonal HAp is usually formed by precipitation from supersaturated solutions at 25–100 °C, while the monoclinic HAp is primarily formed by heating the hexagonal form at 850 °C in air and then cooling to room temperature (Marković et al., 2004).

From a chemical perspective, the composition of biological apatites and synthetic HAp greatly differs from that of stoichiometric apatitic phases due to ion substitutions. Indeed, human bone mineral is composed of non-stoichiometric nanocrystalline apatites with structural imperfections due to co-substituted essential trace elements such as Na, Mg, Zn, Sr, K, F, Cl, Si, and CO₃^{2 −} in crystal lattices (Lin et al., 2011a; Gómez-Morales et al., 2013), in which the cations usually substitute part of Ca^{2 +} ions in apatitic lattice and SiO₄ tetrahedra replace partly the PO₄ tetrahedra, while the anions of F⁻ and Cl⁻ occupy OH sites. As for CO₃^{2 −} ions, they can occupy OH or PO₄ tetrahedra sites to form A- and B-type carbonate apatites, respectively.

HAp has two types of crystal planes with significantly different net charges, positive charges on a and b planes, and negative charges on c planes, respectively. Therefore, the a and b planes tend to attract the molecules with negative charge (e.g., acidic molecules), whereas the c planes prefer to adsorb those with positive charges (e.g., basic molecules) (Uskokovic and Uskokovic, 2011). The elongation of HAp crystals along the c-axis leads to a shift toward more positively charged particles with a higher specificity for adsorption of negatively charged acidic proteins (Uskokovic and Uskokovic, 2011; Kandori et al., 2009). Moreover, the pH value of the soaking medium plays a critical role on the surface charges of HAp. When HAp particles are soaked in mineral acids or bases, negative surface charge is observed in the range of pH 5–8, which becomes even stronger with further increase of pH value (Garcia Rodenas et al., 2005).

In most cases, the morphology of precipitated HAp crystals is hexagonal in shape. It is generally considered that Ca₉(PO₄)₆ clusters with positive charge are the growth unit of HAp crystals (Lin et al., 2011b). Usually, hexagonal HAp crystals that grow along the c-axis are easily obtained because of a strong bond site for Ca₉(PO₄)₆ cluster in [0001] direction, but not in [100] direction. In other words, c-surface is a predominant crystal growth facet compared to a- and b-surfaces (Lin et al., 2011b).

The chemical composition, crystallinity, size, and morphology of the HAp crystals and their aggregates play critical roles in determining their properties and potential applications (Xia et al., 2013; Lin et al., 2013a,c, 2011a,b,c, 2007; Wu et al., 2011a; Zhang et al., 2014; Shen et al., 2012). Nanoscale HAp crystals possess excellent sintering ability due to their high surface energy (Lin et al., 2012), and the HAp nano-ceramics with enhanced mechanical properties can be fabricated using HAp nano-powders as raw materials (Sadat-Shojai et al., 2013). Moreover, HAp nano-bi...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of contributors
• Woodhead Publishing Series in Biomaterials
• Preface
• Part One: Properties and biological response to hydroxyapatite for medical applications
• Part Two: Biomedical applications of hydroxyapatite
• Index